Methods of making threads by plastic deformation of the blank metal, and by rolling in particular, which are now widely applied (see, for example, Pisarevsky M.I., Nakatyvanie tochnykh rezb i shlitsev, 1968, Mashgiz, Moscow-Leningrad), together with such obvious advantages as high capacity, metal saving, and high strength of resulting threads, are also characterized by substantial shortcomings. Among these are first of all a strict tolerance for the blank size intended to prevent jamming the tool when making threads inside bores, and substantial thread-forming forces affecting the tool, especially when making threads inside bores of small diameters.
Known in the art is a method of making an elastic thread having a closed helical cavity inside the thread profile, disclosed in Japan Pat. No. 48-20977, published June 25, 1973, Minoru Kamiya.
The above method comprises preforming a helical groove on the blank and subsequent plastic deformation of its walls by means of a thread-forming tool. The thread-forming tool is positioned to match the top portions of its profile with the top portions of the helical groove. Next, the thread-forming tool is deepended into the top portions of the helical groove lower than the profile thereof, thus providing deformation of its walls which results in obtaining a thread having a closed helical cavity inside the thread profile.
Due to the presence of a closed helical cavity inside the thread profile, the thread thus obtained allows the tolerance for the blank size to be reduced, thread-forming forces affecting the tool to be cut down, and jamming the tool in the process of rolling a female thread to be excluded.
However, the tests have demonstrated the impossibility of obtaining stable elasticity of the thread when making threads in accordance with the above method which is due to the displacement of the cavity relative to the thread profile.
Moreover, when deepening the tool into the top portions of the helical groove, the thread-forming forces affecting the tool are still substantial due to the mode of the metal flow. The metal is extruded in the direction of the thread-forming tool in a plane normal to the axis of the workpiece, thereby filling the profile of the tool.
Known in the art is a thread-rolling tool (U.S. Pat. No. 3,069,940) comprising threading rollers uniformly disposed around the periphery in a body provided with a starting portion. The roller axes are in an angular position relative to the generating line of the body, which angle is equal to the angle of spiral of the resulting thread.
To obtain the above described elastic thread by means of such a tool and by plastic deformation of the walls of the preformed helical groove, it is necessary to position said tool to match the top portions of the profile thereof with the top portions of the profile of the above helical groove. Such positioning may be provided, for example, by means of an MDTW gearing system comprising a machine-tool, a device, a tool, and a workpiece, by adjusting the same to obtain a rigid engagement between the tool and the workpiece. However, such an arrangement substantially complicates practicing the method. Moreover, when using the available MDTW systems, it is rather difficult to ensure a sufficiently rigid engagement between the tool and the workpiece which results in that the cavity inside the profile of the thread to be produced is displaced relative to the axis thereof, and the elasticity parameters of the thread are not stable.